Method of soft x-ray imaging

ABSTRACT

A method for inspecting masks used in x-ray lithography is described. An x-ray lithography mask is placed over a glass surface, followed by exposure of the mask and glass surface to soft x-rays. Portions of the mask absorb the soft x-rays while other portions of the mask, corresponding to circuit elements, allow the soft x-rays to strike the glass surface. The soft x-rays striking the glass surface cause the glass surface to darken, thereby forming an image of the circuit pattern in the glass surface corresponding to the stenciled circuit in the mask. An inspection of the image can reveal any defects in the x-ray lithography mask.

FIELD OF THE INVENTION

This invention relates to the field of x-ray lithography. Moreparticularly, this invention relates to forming images on glass surfaceswith soft x-rays.

BACKGROUND OF THE INVENTION

Currently, integrated circuit (IC) chip patterns are etched out ofsemiconductors using photolithography, ultraviolet lithography, or x-raylithography. Photolithography ultraviolet, and x-ray lithography areprocesses by which a semiconductor is first covered with a layer ofphotosensitive material (i.e. photoresist). This layer is then coveredwith a stencil-like mask in the shape of the desired electric circuit.When the semiconductor is exposed to visible light, ultraviolet light,or x-rays a chemical reaction occurs in the exposed areas of thephotosensitive material or photoresist. Then, depending on the process,either the exposed or unexposed area of the photosensitive material orphotoresist is etched away by acid (i.e. a mixture of H₂ SO₄ and H₂ O₂),leaving a pattern in the shape of the mask.

One drawback with all lithography methods, including x-ray lithography,is that extraordinary care is required to avoid defects in the maskssince masks are used to manufacture millions of chips per mask. Becausethese masks create extremely complex and dense patterns millions oftimes over, inspection of these masks for defects is critical. There aretwo principal types of mask defects: (1) a spot, which is an area thatabsorbs radiation where it should not, and (2) a hole, which is an areathat does not absorb radiation where it should. These defects may beformed in unpatterned areas, patterned areas, or on feature edges. Therecan also be hybrids or combinations of these two principal types ofdefects.

There have been a variety of approaches to x-ray mask inspection. Atypical approach involves inspection of the mask with an electron beamin reflection or transmission. The response of a mask to electrons isnot identical to its response to x-rays, however,and the rate ofinspection is typically slow.

Another approach is exemplified in U.S. Pat. No. 4,718,767 to Hazama. Acopy of a mask is made on a transparent wafer coated with aphotosensitive layer, and after development the resulting pattern on thewafer will be optically inspected. Drawbacks with this method are that(1) the wafers must be developed (i.e. chemical processing of thephotoresist material exposed to the x-ray source) in order to examinethe "spots;" and (2) numerous wafers must be used for multipleinspections throughout a production, which can be relatively expensive.Direct optical inspection is lengthy, tedious, and inaccurate.

Another approach is described in U.S. Pat. No. 5,123,743 to Feldman.Feldman creates two exposures on the same wafer, one through a firstmask onto a positive resist on the wafer, and one through a second,ostensibly identical mask, onto a negative resist on the same wafer.Both masks are either positive or negative. The resulting image on thewafer will be either a completely dark or completely light background,with defects represented as islands or "specks" on the wafer. Drawbackswith this method are that (1) the two mask images (positive andnegative) must be precisely aligned on the wafer-if not, the necessarydark or light background will not be created; (2) the wafers must bedeveloped in order to examine the "spots;" and (3) numerous wafers mustbe used for multiple inspections throughout a production run.

Thus, there remains a need for a simple, inexpensive and accurate methodof inspecting x-ray lithography patterns.

SUMMARY OF THE INVENTION

The present invention contemplates a method of forming an image on aglass surface using soft x-rays, comprising the steps of: a) providing asource of soft x-rays and a glass surface; and b) exposing said glasssurface to soft x-rays from said source of soft x-rays, whereby an imageis formed on said glass surface, said image corresponding to areas onsaid glass surface of exposure to said soft x-rays.

In one embodiment, the glass surface comprises sodium. In anotherembodiment, said source of soft x-rays is a synchrotron. In yet anotherembodiment, said source of soft x-rays is an x-ray tube.

In yet another embodiment, the method of forming an image on a glasssurface further comprises the step of positioning a mask between saidglass surface and said source of soft x-rays prior to said exposingstep, whereby said mask blocks said soft x-rays directed upon said glasssurface.

In yet another embodiment, the method of forming an image on a glasssurface further comprises the steps of a) positioning a splitting meansbetween said glass surface and said source of soft x-rays; and b)splitting said soft x-rays into two coherent beams whereby i) a firstbeam directly strikes said glass surface and ii) a second beam strikesan object and is then reflected toward and strikes said glass surface,whereby said first and second beams discolor said glass surface in sucha way as to form a hologram image of said object. The present inventionalso contemplates a glass surface comprising a hologram formed inaccordance with this process.

In one embodiment said source of soft x-rays is a soft x-ray laser. Inanother embodiment, said source of soft x-rays is an x-ray tube. In yetanother embodiment, said splitting means comprises mirrors.

The present invention also contemplates a method for inspecting asemiconductor mask, comprising the steps of: a) providing a source ofsoft x-rays, a glass surface, and a semiconductor mask for creatingintegrated circuits; b) positioning said semiconductor mask in contactwith said glass surface; c) positioning said source of soft x-rays sothat said semiconductor mask is between said glass surface and saidsource of soft x-rays; d) irradiating said glass surface with softx-rays from said source of soft x-rays whereby said semiconductor maskblocks a portion of said soft x-rays from striking said glass surface sothat regions of said glass surface exposed to said soft x-rays discolorwhile regions of said glass surface not exposed to said soft x-rays donot discolor; and e) inspecting said discoloration on said glass surfacefor defects which correspond to defects in said semiconductor mask. Thepresent invention also contemplates a glass surface comprising an imageof an integrated circuit formed in accordance with this process.

In another embodiment, said step of inspecting said glass surfacefurther comprises the steps of: a) applying a thin film of aphotoelectron-emitting material to said glass surface; b) projecting ablue light from behind said glass surface, through a back side of saidglass surface, through regions of said glass surface not discolored, andthrough said photoelectron-emitting material in order to cause electronemissions from portions of said photon-emitting material correspondingto nondiscolored regions of said glass surface; and c) analyzing saidelectron emissions with a photoelectron microscope.

In one embodiment, said photoelectron-emitting material is cesium. Inanother embodiment, said photoelectron-emitting material is potassium.

The present invention also contemplates a method for forming a hologramin a glass surface comprising the steps of: a) providing a soft x-raylaser source, a glass surface, a splitting means and an object; b)positioning said splitting means between said glass surface and saidsoft x-ray laser source; c) projecting a laser beam from said soft x-raylaser source toward said splitting means; and c) splitting said laserbeam into first and second coherent beams whereby said first beamtravels toward and strikes said glass surface and said second beamreflects off said object and travels toward and strikes said glasssurface whereby a hologram image of said object is formed on said glasssurface. The present invention also contemplates a glass surfacecomprising a hologram formed in accordance with this method.

In one embodiment, said splitting means comprises mirrors. In yetanother embodiment, said splitting means comprises a surface layer ofMolybdenum-Silicon.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a block diagram of the elements of the presentinvention used to inspect x-ray masks.

FIG. 2 illustrates a block diagram of a preferred method of inspectingthe resulting image on the glass surface.

FIG. 3 illustrates a device for photoelectron microscopy.

FIG. 4 illustrates a block diagram of the elements of the presentinvention used to generate a hologram.

FIG. 5a illustrates a glass slide with the left half exposed to softx-rays.

FIG. 5b illustrates a glass slide with the word "Hawai'i" imaged intothe glass by using a mask containing that word.

FIG. 6 illustrates the photoelectron spectra of the dominant elements(Si and O) and the most prominent metal impurity Na in the glass slidesused in the present invention.

FIG. 7 illustrates the spectral absorbance of the darkened areas andindicates greater optical absorbance in the green and blue regions ofvisible light.

FIG. 8 illustrates the relative atomic concentration of sodium of thedarkened areas as a function of increasing time of exposure to the softx-rays.

GENERAL DESCRIPTION OF THE INVENTION

The present invention describes a method for creating an image on aglass surface by exposing the glass surface to soft x-rays. X-rays areregarded as part of the electromagnetic spectrum having wavelengthsbetween 0.01 and 10 nanometers. X-rays generated from x-ray tubestypically have wavelengths between 0.01 and 2 nanometers, whereas highenergy accelerators can generate x-rays with wavelengths shorter than0.01 nanometers and synchrotron sources can generate x-rays havingwavelengths longer than 2 nanometers. Soft x-rays for lithography arex-rays having a wavelength of between 0.8 and 2 nanometers. See B.El-Kareh,Fundamentals of Semiconductor Processing Technology, p. 234(1995).

The present invention demonstrates that, due to a darkening of the glasssurface when exposed to soft x-rays, images can be generated on theglass surface by controlling the pattern of exposure on the glasssurface. In the semiconductor field, this can be accomplished byoverlaying a mask on the glass surface so that an inverse image of themask is generated on the glass surface. By "inverse image" it is meantthat those areas exposed to the light will darken while those areascovered by the mask remain unchanged.

In a preferred embodiment of the present invention, an x-ray mask havinga stencil image of a circuit pattern can be placed between a glasssurface and a soft x-ray source. The x-ray mask can be placed either inclose proximity to the glass surface or in contact with the glasssurface, depending on the application. By "stencil image" it is meantthat the x-ray mask has gaps in the shape of circuit lines which aretransparent to soft x-rays, allowing the soft x-rays to pass through.The remaining portions of the x-ray mask block or absorb the softx-rays. In this manner, an image of the x-ray mask's circuit pattern isformed on the glass surface as a discolored region. These discoloredregions can then be inspected for defects in the x-ray mask.

This embodiment of the present invention provides a method that allowsfor greater accuracy than inspecting the masks themselves, becausedefects transferred from the masks to the semiconductors may not beeasily identified on the masks themselves. It is also cheaper, quickerand more efficient than inspecting resulting ICs because it does notrequire the use of semiconductor wafers or the development of aphotosensitive layer.

In an alternative embodiment, soft x-ray diffraction and scatteringpatterns can be detected. For example, holograms can be generated inglass surfaces by taking advantage of glass surface discolorationresulting from light waves diffracted off an object. In this alternativepreferred embodiment, the glass surface could be glass film deposited ona transparent substrate. In this case, the surface need not be flat.

In this alternative embodiment, a beam-splitting means (i.e. mirrors) ispositioned between a glass surface and a soft x-ray laser for dividingthe laser beam into two beams. The first beam continues directly towardthe glass surface while the second beam is reflected off an object andthen toward the glass surface. The interference image formed in theglass surface by these two beams form a hologram.

The image of the object can be reconstructed from the hologram byshining blue light from the back of the glass surface.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The following description serves to illustrate particular embodimentsand aspects of the present invention and is not to be construed aslimiting the scope thereof.

In considering the method of soft x-ray imaging on glass surfaces asdescribed in this application, certain definitions are helpful."Adjacent to" means in close proximity (of one object to another)without contact. "Blue light" is light having a wavelength ofapproximately 450 to 490 nanometers. A "boat" refers to any means forholding a specimen of the photoelectron emitter while the emitter isbeing heated to form vapors of the emitter. "Coherent beams" are beamshaving a fixed phase relation with respect to one another. In otherwords, the phase of the waveform corresponding to the beams, remainsconstant during the resolving time of an observer. "Defects" areanomalous conditions in a semiconductor mask such as holes (i.e. clearareas in the mask) and spots (i.e. opaque areas in the mask). "Discolor"and "discoloration" refers to the color change in the glass surface dueto exposure to soft x-rays. In the example provided below, the changewas from a clear glass to glass having a brown-opaque color. An"evaporator chamber" is a vacuum chamber wherein a photoelectron emittersuch as cesium or potassium is heated, causing vapors of thephotoelectron emitter to be formed. A coating of the photoelectronemitter can be formed on a glass surface by placing the glass surface inthe chamber such that the vapors contact the glass surface. "Exposing"means allowing light to strike the exposed surface of an object, such asglass. A "gate valve" refers to a means for sealing the evaporationchamber of the system illustrated in FIG. 3 (ie. the region of thesystem to the right of the gate valve in FIG. 3) from the region of thesystem to the left of the gate valve.

"Holograms" are three-dimensional images of an object formed byrecording, on a two dimensional surface, the pattern of interferenceformed by coherent beams. "Integrated circuits" are circuits whosecomponents and connecting wires are made by processing distinct areas ofa chip of semiconductor material. A "linear motion manipulator" refersto any means (e.g a mechanically-driven platform) or for allowing a userto control placement of the glass surface in the device illustrated inFIG. 3. A "mask" is a stencil for creating semiconductors whereincertain regions of the mask allow light having a specific wavelength(i.e. x-rays) to pass through while other regions block or absorb thelight. "Photoelectron-emitting materials" are materials (such as cesiumand potassium) that emit photoelectrons when excited by an energy sourcesuch as blue light. "Semiconductor masks" are stencils in the shape of acircuit pattern that are used to form circuit lines on a semiconductor.It should be noted that in the field of x-ray lithography, semiconductormasks are masks that allow a portion of x-rays to pass through whileabsorbing or blocking another portion of x-rays projected upon thesemiconductor mask. A "shutter" is a means for controlling the amount ofphotoelectron vapor that is allowed to contact the glass surface. "Softx-rays" for lithography are x-rays having a wavelength of between 0.8and 2 nanometers. A "splitting means" refers to any means (e.g. mirrors)for splitting x-rays into two distinct and coherent beams. By "twodistinct and coherent beams" it is meant that the x-rays are dividedinto two separate x-ray beams with each beam travelling at a differentpath from each other and each beam having a fixed phase relation withrespect to the other. "Synchrotrons" are high intensity x-ray sources(i.e. one such device can be found at the Lawrence Berkeley Laboratoryin Berkeley, California). "X-ray tubes" are electronic devices forgenerating x-rays. This is accomplished by accelerating electrons to ahigh velocity with an electrostatic field and then suddenly stoppingthem by collision with a solid body. X-rays are radiated from the pointsof collision.

The present invention takes advantage of a conventional glass surface'sreaction to soft x-ray radiation. Glass surfaces take on a brownishcolor when exposed to soft x-rays, with the intensity of the colorincreasing as the exposure time or intensity of the soft x-rayirradiation increases. This discoloration is stable and durable, meaningthat it remains constant after the irradiation has stopped.

While an understanding of the mechanism is not necessary for successfuluse of the invention, it is believed that sodium is the dominant metalimpurity that causes the discoloration. An analysis of the relativeatomic concentration of the impurity shows an increase of sodium withincreasing exposure of the glass surface to the soft x-rays. The resultssuggest that the color-center sites (i.e. the regions of brownish color)involve trapped photoelectrons near the surface region of the glass,which causes -mobile sodium ions to move toward the exposed glasssurface. The discoloration for a given length of exposure depends on theintensity of the incident soft x-rays and the relative concentration ofsodium in the glass. For soft x-ray lithography, there is presumedly anoptimal concentration of sodium that can be put into the glass whichwould optimize the discoloration for a given soft x-ray intensity andexposure time.

Of course, it is possible that other impurities or elements, other thansodium, may be equally effective at reacting with soft x-rays to producethe desired discoloration.

MAKING IMAGES ON GLASS

Because of the short wavelength of soft x-rays, images of small featuresformed on glass surfaces can be resolved. Such precision is well suitedfor inspecting x-ray lithography masks which create extremely smallcircuits having line widths less than or equal to 0.10 micrometers.

Referring to FIG. 1, an x-ray lithography mask 2 having a stencil image(i.e. a circuit pattern) is placed between a soft x-ray source 4 and aglass surface 6. One method is to place the mask in contact with theglass surface (as in contact photolithography or contactmicroradiography). In an alternative embodiment, the x-ray lithographymask may be placed adjacent to the glass surface. This embodiment haspractical application in x-ray reduction lithography wherein a largerx-ray mask can be used to generate smaller resulting images using curvedmirrors. See Synchrotron Radiation News, Vol. 4, No. 2, pp. 13-15(1991).

In operation, as soft x-rays are emitted, the x-ray lithography mask 2obstructs soft x-rays from striking the glass surface 6, thereby forminga pattern corresponding to the stencil image in the x-ray lithographymask 2. The resulting discoloration of the glass surface 6 can beexamined for any defects. Examination of the resulting image provides asuperior inspection method to direct examination of the mask, becausethe image is created in the same manner as an IC would be created fromthe mask.

Referring now to FIG. 2, examination of the resulting image in the glasssurface 6 (created by the operation described in the precedingparagraph) can be accomplished by photoelectron microscopy. See O. HayesGriffith et al., Proc. Natl. Acad. Sci. U.S.A., Vol. 86, pp. 1826-1830(1989). A thin film of a photoelectro-nemitting material 8 (e.g. cesiumor potassium) is applied to the glass surface 6, after which, visiblelight of a specific frequency (e.g. blue light having a wavelength ofapproximately 450 to 490 nanometers) projected from a light source 10through the back of the glass surface 6, causes electrons 11 to beemitted from the photoelectron-emitting material 8 in a patterncorresponding to the non-discolored regions of the glass. Thephotoelectron-emitting material 8, corresponding to the non-darkenedregions, emits electrons because the non-discolored regions allow thelight to pass through the glass surface 6 to the photoelectron-emittingmaterial 8 thus producing electron emissions. The emitted electronpattern is then analyzed using a photoelectron microscope 12. Defectssuch as pinholes in the semiconductor mask 2 (shown in FIG. 1) whichcreated the image in the glass surface 6 are detected as gaps in thex-ray mask, which allow soft x-ray photons to pass through. It isfurther contemplated that a computer can be coupled to the photoelectronmicroscope to digitize an image corresponding to the emitted electronpattern, so that the computer can compare the resulting image with asoftware representation of an accurate mask.

FIG. 3 illustrates a photoelectron microscopy device used for viewingthe image of the x-ray mask created on the glass surface 6 (see above).Starting from the right of the device in FIG. 3, the glass surface(sample) 6 is placed, with the glass surface 6 face down, in theevaporator chamber 26 using a linear motion manipulator 28 (controlledby manipulators 44). A boat 30, containing a photoelectron emitter suchas cesium or potassium, is located beneath the glass surface 6, whereinthe emitter is heated using the electrodes 34 and 36. As the emitter isheated, vapors rise from the boat 30 and pass through a shutter 32 whichcontrols the amount of emitter deposition. As the vapors rise, a thinfilm is deposited on the glass surface 6. A film thickness monitor 38,is used to determine the film thickness deposited on the glass surface6. Although the present invention will work with a variety of differentdeposition thicknesses, a preferred deposition thickness is 10nanometers.

After deposition, the glass surface 6 is transferred to thephotoelectron microscope chamber 42 through the gate valve 40 using thelinear motion manipulator 28. The glass surface 6 is placed between anelectronic lens system 46 and optical lenses 52. The electronic lenssystem allows for user controlled magnification and focusing of theglass surface. Optical lenses 52 assist in focusing blue light fromlight source 48, through window 50, through lenses 52, and toward theglass surface 6. When the blue light strikes the glass surface 6,photoelectrons from the photoelectron emitter deposited on the glasssurface 6, are emitted from the glass surface 6. These photoelectronsare then magnified by the electron lens system 46 and imaged by theimage intensifier/TV system 54 for viewing by a processor or user (notshown). The image intensifier/TV system 54 provides for viewing of theglass sample by a user.

An alternative embodiment of the present invention provides for thedetection of diffraction and scattering patterns, which can be used, forexample, to generate holograms on glass surfaces with high resolution.By incorporating x-ray laser beams, the present invention can also beused to generate holograms on glass surfaces.

FIG. 4 illustrates a block diagram of the elements of the presentinvention used in the hologram embodiment. Specifically, a soft x-raylaser source 14 directs an x-ray laser beam toward the glass surface 16.A splitting means 18 (i.e. special mirrors with, for example,Molybdenum-Silicon multilayer coatings) splits the laser beam into twobeams 20 and 22. The first beam 20 continues directly toward the glasssurface 16. The second beam 22 is reflected off an object 24 and backtoward the glass surface 16. The resulting discoloration on the glasssurface 16 will be able to generate a hologram. As shown in spectralabsorbance measurements (FIG. 7), the resulting discoloration on theglass surface has greater absorption of blue light (having a wavelengthof approximately 450 to 490 nanometers) than red light (having awavelength of approximately 630 to 750 nanometers). The image of theobject can be reconstructed from the hologram by shining blue lightthrough the glass surface. The magnification is proportional to theratio of the wavelength of the blue light to that of the soft x-rays.See R. D. Guenther, Modern Optics, John Wiley & Sons, New York, Chapter12 (1990).

EXAMPLE 1

Thin glass plates, at room temperature, were cut from ordinarymicroscope slides and exposed to soft x-rays from a 300-watt sourceElectron Spectroscopy Chemical Analysis System (ESCA) Model 5100,manufactured by Perkin-Elmer: Eden Prairie, MN!, utilizing a magnesiumtarget. By magnesium target, it is meant that a sample of pure magnesium(the "target") is bombarded with electrons resulting in the generationof soft x-rays emitted from, and having characteristics of, the target.By way of illustration, soft x-rays from a magnesium source have lowerenergy than those from aluminum. In this experiment, the soft x-rayemissions had a wavelength of approximately 1 nanometer. Although othersoft x-rays wavelengths were not utilized, soft x-rays having otherwavelengths should also work provided the wavelengths are sufficientlylong so that absorption of the incident x-rays can occur. A preferredwavelength is 0.8-2 nanometers, while a most preferred wavelength is0.83-0.99 nanometers.

X-ray photoelectron spectroscopy (XPS) was the technique used toidentify the atomic elements on the surface of the glass plates. Toidentify elements such as sodium, x-rays from an aluminum target wereused. X-rays from a target of pure aluminum are better than x-rays froma magnesium source in the XPS study of sodium because of their higherenergy.

FIG. 5a illustrates two halves of a microscope slide, with the left sidehaving been exposed to soft x-rays from a magnesium target. Thisillustrates the coloration difference between an exposed slide regionand a nonexposed slide region. FIG. 5b illustrates a microscope slidewith the word "Hawai'i" imaged on its surface. The image was formed byplacing a mask with the word "Hawai'i" in stencil over a glass slide andexposing the slide to soft x-rays. The mask can be made of any materialable to absorb soft x-rays, thereby preventing them from penetrating themask. In the present example, although a variety of differentthicknesses could have been utilized, the mask was made using a thinmetal foil of molybdenum, approximately5 milliinches thick (125microns). The image of the word "Hawai'i" was stable and durable.Because of the short absorption depth of soft x-rays in glass, thedarkening effect was limited to the surface region of the glass.

In examining the glass surfaces after exposure to soft x-rays, thesamples were placed in the vacuum chamber of the ESCA for elementalanalysis with x-rays from an aluminum target. An aluminum target wasused here because the radiation emitted from it has higher energy thanthe magnesium target used to generate the discoloration. This higherenergy was necessary to create an adequate reaction with the predictedsodium ions in the glass slide, because aluminum provides higher energyx-ray photons which excite the lowest energy level electron from sodium.This excitation enables the determination of the relative concentrationof sodium at the glass surface region.

The photoelectron spectra of the dominant elements (Silicon and Oxygen)and the most prominent metal impurity (Na) in the glass slides used areshown in FIG. 6. Traces of other metals (Mg and K) and carbon were alsodetected. The relative atomic concentration of sodium was determinedfrom the analysis of the photoelectron spectra before and after exposureto the soft x-rays from a 300-watt source with a magnesium target.Measurements were made after successive exposures at room temperature.The darkening of the glass sample surface was more noticeable withlonger periods of exposure.

Spectral absorption measurements were made on the soft x-ray-induceddarkened area of the glass surface. The absorbance was determined bymeasuring the transmittance of the darkened area and that of the cleararea in the spectral range of 300-600nm. The spectral absorbance resultsare shown in FIG. 7. The results indicate greater optical absorbance inthe green (having a wavelength of approximately 490-570 nanometers) andblue (having a wavelength of approximately 450 to 490 nanometers)regions of the spectrum. This is consistent with the observed browncolor of the exposed regions of the glass surfaces.

FIG. 8 shows the relative atomic concentration of sodium in the darkenedarea as a function of the increasing time of exposure to the softx-rays. The data indicate that a higher concentration of the sodiumappears in the surface region of the darkened area after a longer periodof exposure. While an understanding is not necessary, it is believedthat a possible explanation for this enhancement is that trappedphotoelectrons near the top surface layer of the glass attract themobile sodium ions, thereby causing more sodium to appear in the surfaceregion. It was assumed that some of the photoelectrons generated by thesoft x-rays were trapped at the sites of oxygen vacancies (missingoxygen atoms in the Si-O glass structure). The presence of sodium ionsis expected because the mobile ions can account for the electricalconductivity of glass containing sodium. The mobile sodium ions couldcome from the photo-dissociation of the impurity centers in the glass.Photo-dissociation is a process of breaking the molecular structure intopositive and negative ions by using light (in our case, x-rays). Thetrapped photoelectron sites with the surrounding sodium ions are thepossible color centers, and the selective absorption of visible lightaccounts for the observed brown color.

EEAMPLE 2

In this example, a thin glass plate, at room temperature, was again cutfrom an ordinary microscope slide and exposed to soft x-rays from thesame 300-watt source used in Example 1. A mask comprising a 1.5millimeter thick nickel foil having square holes approximately 75 micronon a side (Metrigraphics, Wilmington, MA), was placed over, and incontact with, the glass plate surface. The mask and glass plate werethen exposed to the soft x-rays. The soft x-rays passing through thesquare holes darkened the exposed glass surface such that the darkenedareas were clearly visible under a microscope.

From the above it is clear that the present invention provides for asimple, inexpensive, and accurate method of imaging x-ray lithographypatterns for x-ray mask inspections.

We claim:
 1. A method of forming an image on a glass surface using soft x-rays, comprising the steps of:a. providing a source of soft x rays, a mask and a glass surface; b. positioning said mask between said glass surface and said source of soft x-rays; and c. exposing said glass surface to soft x-rays from said source of soft x-rays, whereby said mask blocks a portion of said soft x-rays and an image is formed on said glass surface, said image corresponding to areas on said glass surface exposed to said soft x-rays.
 2. The method of claim 1 wherein said glass comprises sodium.
 3. The method of claim 1 wherein said source of soft x-rays is a synchrotron.
 4. The method of claim 1 wherein said source of soft x-rays is an x-ray tube.
 5. A method of forming an image on a glass surface using soft x-rays, comprising the steps of:a. providing a source of soft x rays, a mask and a glass surface; b. positioning said mask between said glass surface and said source of soft x-ras; c. exposing said glass surface to soft x-rays from said source of soft x-rays, whereby said mask blocks a portion of said soft x-rays and areas on said glass surface exposed to said soft x-rays discolor and other areas do not discolor; d. applying a thin film of a photoelectron-emitting material to said glass surface; and e. projecting a blue light from behind said glass surface through said photoelectron-emitting material in order to cause electron emissions from portions of said photon-emitting material.
 6. The method of claim 5 wherein said photoelectron-emitting material is cesium.
 7. The method of claim 5 wherein said photoelectron-emitting material is potassium.
 8. A method for inspecting a semiconductor mask, comprising the steps of:a. providing a source of soft x-rays, a glass surface, and a semiconductor mask for creating integrated circuits; b. positioning said semiconductor mask in contact with said glass surface; c. positioning said source of soft x-rays so that said semiconductor mask is between said glass surface and said source of soft x-rays; d. irradiating said glass surface with soft x-rays from said source of soft x-rays whereby said semiconductor mask blocks a portion of said soft x-rays from striking said glass surface so that regions of said glass surface exposed to said soft x-rays discolor while regions of said glass surface not exposed to said soft x-rays do not discolor; and e. inspecting said discoloration on said glass surface for defects which correspond to defects in said semiconductor mask.
 9. The method of claim 8, wherein said glass comprises sodium.
 10. The method of claim 8, wherein said source of soft x-rays is a synchrotron.
 11. The method of claim 8, wherein said source of soft x-rays is an x-ray tube. 